This invention relates to the field of tacky ethylene-vinyl acetate copolymers and to the tacky partially-hydrolyzed ethylene-vinyl acetate copolymers, and more particularly, to processes for comminuting these copolymers and converting them into dry powder compositions.
2. Description of the Prior Art
High pressure ethylene-vinyl acetate (EVA) copolymers are a well-known class of thermoplastic resins. Copolymers containing up to about 60 weight percent of vinyl acetate are now available commercially via a modified high pressure polyethylene process operating at 1000-2000 atmospheres, and compositions containing as much as 85 weight percent vinyl acetate have been made experimentally by the high pressure process. EVA resins containing up to about 35 weight percent of vinyl acetate are readily handled in the form of pellets, yet even resins with a vinyl acetate content in the upper part of this range tend to stick together under the pressure of their own weight, especially under hot storage conditions. Copolymers containing between about 35 and about 60 weight percent of vinyl acetate can also be pelletized, but the pellets tend to be tacky and coalesce increasingly at higher vinyl acetate contents in this range. At about 40 to 45 weight percent vinyl acetate, the pellets maintain their identity, but often partially fuse into bulky masses resembling bunches of grapes. At 50 to 60 weight percent vinyl acetate content, the pellets soon lose their identity and these resins normally assume the shape of their container by cold flow, and consequently are only available as solid blocks. Copolymers containing 35-85 weight percent vinyl acetate can be produced with melt flow rates below 5 (Condition B, ASTM D1238) and in most cases as low as 0.2, when finished by thermal treatment by known methods, e.g. U.S. Pat. No. 3,968,091.
U.S. Pat. No. 3,517,083 discloses that EVA resins containing 15 to 60 weight percent vinyl acetate may be used as impact modifiers in rigid polyvinyl chloride (PVC) formulations, and that EVA copolymers containing 60 to 85 weight percent vinyl acetate are useful in producing flexible blends with PVC. However, a serious impediment in blending high pressure EVA into PVC is the aforesaid physical form of the EVA resins. To blend even EVA pellets into PVC, which is normally supplied as a powder, requires the expenditure of considerable energy and introduces an undesirable heat history into the PVC (e.g., see Plastics Engineering, April 1967, p. 47; Plastics Technology, July 1975, P. 50). Blending of the fused pellets characteristic of the EVA resins containing above about 50 weight percent vinyl acetate would obviously be still more difficult than blending free pellets.
Accordingly, to facilitate the blending of high pressure EVA copolymers into PVC powder, and into other pulverulent polymers as well, it would be highly desirable to have the EVA copolymers in the powder form. Dry blending of PVC powder and the EVA powder could then be readily accomplished at little expenditure of energy and without imparting an undesirable heat history to the heat-sensitive PVC due to the blending operation itself. Prior to the present invention, however, tacky EVA copolymers containing from 35 up to about 85 weight percent vinyl acetate, in particular those made by the high pressure process, have not been available as dry powders, although copolymers containing 60 to 70 weight percent vinyl acetate and made by emulsion polymerization can be recovered as powders suitable for dry blending. However, emulsion EVA resins of lower vinyl acetate content are not available and, since emulsion polymerization is comparatively a slower process than the high pressure process, it would be advantageous to synthesize the EVA resins by the rapid high pressure process and convert this broad range of EVA resins to powders by an equally rapid process.
Hydrolyzed ethylene-vinyl acetate (HEVA) copolymers, particularly the so-called partially hydrolyzed copolymers, herein defined as EVA resins originally contain about 35 to about 85 weight percent of vinyl acetate have been generally known for may years. They resemble the high pressure EVA resins in being inherently tacky materials but present certain advantages by virtue of their hydroxyl functionality, e.g. enhanced adhesion to various substrates, additional cross-linkability, and have superior heat and mill stability as compared to EVA. In addition, as disclosed in this invention, those less than 50% hydrolyzed also function as impact modifiers in rigid PVC formulations, whereas the substantially fully hydrolyzed EVA resins do not.
According to U.S. Pat. No. 3,474,058, films cast from EVA copolymer can be given improved slip and antiblocking properties by incorporating therein small amounts of a metal salt of a fatty acid having from about 12 to 22 carbon atoms, for example, calcium stearate, in combination with a fatty acid amide, for example, stearyl amide. Commonly assigned, copending U.S. patent application Ser. No. 783,527, filed Apr. 1, 1977 discloses and claims a vinyl acetate ethylene elastomeric copolymer having improved anti-block characteristics imparted thereto by the presence of a small amount of a hydrogenated glyceride or mixture of glycerides such as refined hydrogenated tallow glyceride. In addition to improved anti-block characteristics, the copolymer can also be provided with improved processing release properties by combining the glyceride or mixture of glycerides with stearic acid. U.S. Pat. No. 3,468,827 describes a method for reducing the corrosive nature of EVA copolymers for steel and other materials with which these resins come into contact during processing, carried out by incorporating into the resins a salt of fatty acid such as calcium stearate.